In medical roentgen diagnosis it is very important that the patient is exposed to a radiation dose that is as small as possible and that, given that small dose, the radiologist has a diagnostically useful image. The first requirement, low dose, for the patient leads to the necessity to have the right diagnostic image with the first shot and repeat exposures due to diagnostically meaningless images has to be avoided. The second requirement, getting a readable image with that low dose, leads to the necessity that the exposure time can be adjusted for various patients, e.g. it must be possible to get a good chest image of very slim patient as well as of an obese patient without having to repeat exposure.
The manufacturers of roentgen equipment therefore introduced a phototimer. An X-ray apparatus the phototimer is placed either in the plane of the cassette or under the cassette depending on the intended use of the X-ray apparatus, e.g., in X-ray apparatus intended for mammography or thorax imaging, the phototimer is placed under (or behind) the cassette. An X-ray apparatus, wherein the phototimer is placed under the cassette operates basically as follows. An X-ray detector is placed under (i.e. on the side of the cassette farthest away from the X-ray tube) the cassette containing the film to be exposed to detect what X-ray dose is not absorbed by the patient and cassette. For every type of examination a threshold value for the dose reaching the phototimer is pre-set. This threshold value is determined by the parameters of the imaging chain (speed of the screens, speed of the film, contrast of the film, processing conditions of the film, etc.) and the parameters of the exposure (X-ray absorption of the body parts to be examined, desired film density and contrast for accurate reading of the picture) The exposure system makes sure that as long as the dose on the phototimer has not reached the pre-set threshold value the X-ray tube will emit X-rays. After a certain time of exposure the phototimer will shut down the X-ray tube. The time of exposure is thus largely governed by the absorption in the patient, in the cassette, in the film and in the intensifying screens. As long as the absorption of in the cassette, in the film and in the intensifying screens, is identical from exposure to exposure, the inclusion of the phototimer fulfils its purpose: variations in roentgen absorption of the patient are taken in account so as to have the same density and contrast on the film irrespective of the X-ray absorption of the patient.
In a “cassette assembly”, comprising a cassette, a film, and an intensifying screen, the intensifying screen itself absorbs most of the energy. Thus variations in the absorption of the screen, translate changes to the time the X-ray tube is operated that are not related to the patient, so it is of high importance to produce intensifying screens with very low variation, between batches as within batches, in X-ray absorption. This is especially so for screens having very high X-ray absorption, which is most likely to occur in those diagnostic situations where low energy X-rays are used. This is, e.g., so in mammography, where mostly X-rays with an energy of 27 kVp are used. Cassettes loaded with film and screen(s) absorb then 80 to 90% of the X-rays. When the cassette loaded with film and screen(s) has and X-ray absorption of e.g. 90%, variations of 1% in that absorption will cause a phototimer to keep the X-ray tube open for a time that varies around 10%. This points out that, for high X-ray absorption, small absorption variations of the intensifying screens are enhanced by the phototimer system, so that exposure time variations can be an order of magnitude larger than the variations of the absorption of the system. The variation of the exposure time has an effect on the density of the film that, especially in mammography, are much larger than tolerated by regulations as, e.g., ACR 2002 (US-Norm). Therefor measures for diminishing the variations of the X-ray absorption of a cassette loaded with film and screen(s) to an acceptable low level is very desirable.